gating into a habitat which it is best suited to 

 exploit (e.g., Nilsson 1967). In this respect, inter- 

 active segregation of juvenile coho salmon and 

 steelhead enables the surface and drift forager, 

 coho salmon, to inhabit pools and the more bot- 

 tom-oriented steelhead to occupy riffles. Johnson 

 and Ringler (1980) pointed out that in sympatry 

 with coho salmon, steelhead are found in areas 

 (riffles) where the standing crop of benthic in- 

 vertebrates is the greatest, an obvious advantage 

 in conjunction with their benthic habits. Con- 

 versely, coho salmon which feed predominantly 

 from the drift presumably have a relatively long 

 time interval in which to observe drifting prey 

 because of decreased water velocities in pools. 



Johnson and Ringler (1980) demonstrated that 

 in sympatry the diet of subyearling coho salmon 

 was closely associated with the composition of the 

 drift, and the diet of subyearling steelhead was 

 more closely associated with the bottom fauna. 

 They found a low degree of overlap in the diurnal 

 summer diet of coho salmon and steelhead due to 

 the utilization of drifting terrestrial invertebrates 

 by coho salmon and benthic invertebrates by steel- 

 head. They speculated that since the composition 

 of invertebrates on or within the substratum of a 

 stream is much more stable over a 24-h period 

 than the composition of the invertebrates drifting 

 over it, that the diet of the drift feeder (coho 

 salmon) would be more variable than the diet of 

 the benthic forager (steelhead) over a 24-h period. 

 The purpose of this study was to test this hypo- 

 thesis while also gathering information sufficient 

 to determine diel feeding periodicity, daily meal, 

 and daily ration of juvenile coho salmon and 

 steelhead. 



Methods 



Subyearling coho salmon and steelhead were 

 collected from a 200 m section of Orwell Brook, 

 Oswego County, N.Y. The stream discharges into 

 the Salmon River which empties into Mexico Bay 

 in the southeastern portion of Lake Ontario. 

 Orwell Brook is a high quality spawning and 

 nursery stream for salmonids migrating from 

 Lake Ontario (Johnson 1980). The 200 m stream 

 section generally consisted of a series of pools, 

 runs, and riffles. The surface substrate consisted 

 of gravel and pebbles. Maximum and minimum 

 water temperatures recorded during the study 

 period were 21° C at 1600 h (31° C air temperature) 

 and 16° C at 0400 h (17° C air temperature). 



The study was carried out in July 1979 since 

 previous studies had shown that subyearling coho 

 salmon and steelhead are most abundant in Or- 

 well Brook at this time (Johnson 1980). Maximum 

 numbers of coho salmon and steelhead in the 

 stream were desired in order to facilitate col- 

 lections and to prevent depletion of the popula- 

 tions of each species. Fish were collected at 4-h 

 intervals commencing at 0800 h on 13 July and 

 ending at 0400 h on 14 July 1979. During the 

 study period sunrise occurred at 0436 h and sunset 

 atl943 he.s.t. Aminimumof20individualsofeach 

 species on 13 July 1979 was collected during each 

 4-h interval. Fish were collected with a 3 m seine, 

 slit, and immediately placed in 10% Formalin^ in 

 order to halt digestive processes. Prior to the 

 removal of their digestive tracts all fish were 

 weighed (grams) and measured (millimeters total 

 length, TL). Correction factors were used to ac- 

 count for weight gain and length shrinkage 

 caused by preservation (Parker 1963; Stauffer^). 

 Dietary items were identified to the family level 

 for aquatic invertebrates and to the ordinal level 

 for terrestrial invertebrates. 



Dry weight estimates were obtained for in- 

 dividuals of each family of aquatic prey and each 

 order of terrestrial prey (invertebrate taxa which 

 had no life state occurring in the aquatic environ- 

 ment) to determine the relative contribution of 

 food organisms in the fishes' diet. A representa- 

 tive number of individuals of each prey taxa 

 (usually 10) were used to derive a dry weight 

 estimate. Food items were placed in small pre- 

 weighed pans and then dried at 105° C for 24 h. 

 The pans were then placed in a desiccator for 6 h, 

 and reweighed. The dry weight of the organisms in 

 the pan was divided by the number of individuals 

 in the pan giving an average dry weight estimate 

 of an individual of that taxa. Dry weight deter- 

 minations were used to estimate diet composition 

 for both coho salmon and steelhead for each 4-h 

 interval. In addition, dry weight estimates for 

 each taxon were summed for the 24-h period to 

 derive an estimate of diel diet composition. 



To examine diel patterns of food consumption of 

 both coho salmon and steelhead the total dry 

 weight of the stomach contents per dry weight of 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



^Stauffer, T. M. 1971. Salmon eggs as food for stream 

 salmonids and sculpins. Mich. Dep. Nat. Resour. Fish. Res. 

 Rep. 1776, 10 p. Institute for Fisheries Research, Museums 

 Annex Building. Ann Arbor, MI 48109. 



371 



